(a) Field of the Invention
The present invention relates to a lithium secondary battery, and in particular, to a separator for an electrode assembly.
(b) Description of Related Art
Generally, batteries are largely classified into a secondary battery which is capable of being repeatedly charged and discharged, and a primary battery which is not rechargeable and is disposed of after it is depleted. The battery is formed with various shapes, such as a cylindrical shape, a prismatic shape, and a pouch shape, depending upon the profile of the target appliance.
The secondary battery has an electrode assembly in which a negative plate and a positive plate are spirally wound to form of a “jelly-roll,” while interposing a separator therebetween; a can mounting the electrode assembly therein together with an electrolyte; and a cap assembly fitted to the top of the can.
The positive plate of the electrode assembly is electrically connected to the cap assembly via an anode lead, and the negative plate thereof to the can via a cathode lead.
With the lithium ion secondary battery, lithium composite oxide is used as the positive active material to form the positive plate, and carbon is used as the negative active material to form the negative plate. The charging and the discharging occur due to the migration of lithium ions between the anode and the cathode.
The lithium ion secondary battery involves high voltage and high battery capacity, and hence, exhibits excellent performance characteristics for usage as a power supply in portable electronics. However, as an organic electrolyte is used, battery safety is problematic. In order to solve such a problem, there is a need for a separator having performance characteristics differentiated from those of other anhydrous secondary mini batteries. That is, it is very important in enhancing battery capacity and improving battery safety to provide an optimum separator for the battery design system, taking into account such characteristics as chemical stability, thickness, mechanical strength, and current breakage.
The separator basically separates the positive and the negative plates from each other, and maintains high ionic conductivity by absorbing an electrolyte needed for the battery reaction. Particularly in the case of the lithium ion secondary battery, the separator should also prevent the migration of unwanted materials caused by battery reaction, and secure battery safety when any abnormality occurs.
Furthermore, the separator inhibits the thickness expansion of the battery that occurs during the repeated cycles of charging and discharging.
The thickness expansion of the battery due to the repeated cycles of charging and discharging seriously deteriorates the battery reliability. In order to solve such a problem, a battery has been designed to inhibit gas generation therein by removing fine particles from the active material or using an additive in the electrolyte. However, the inhibition of thickness expansion by removing the fine particles from the active material or using an additive in the electrolyte is limited.
Furthermore, as lithium ion secondary batteries have been developed having a high capacity and a thin and flat structure, the separator is made thinner so that it cannot effectively absorb the expansion of the active material due to the repeated cycles of charging and discharging, thereby accelerating the thickness expansion.